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When I was learning DC-DC converters, I think they are a "DC version" of transformer (which is for AC-AC). So when I was designing a 100-30V flyback converter, my suggestion is: if a user inputs a DC 100V source, he gets a 30V output; if he inputs a DC 50V source, he gets a 15V. Just like how transformers act with AC, which have a fixed output/input ratio.

And someone then told me: no. If he inputs a 100V he gets 30V output; if he inputs a 50V, he should still get a 30V; even if he inputs a 15V, he still have to get a 30V. (We were talking about flyback converter. I'm not sure what about buck/boost converter. ) This is like a DC power supply (or it is), with which you want a fixed output.

So I'm curious and a little confused now about the actual situation. Do manufacturers always make DC-DC converters with fixed output voltage? Or with fixed output/input ratio? Or both are possible? In this case, is one of them are more often to use then the other?

The "someone" was like "How dumb are you! How can you even don't know this! " while telling me the story. I think it's more possible what he said makes more sense, because I'm really stupid. But what confused me is: if the actual situation is we never design DC-DC converters, like buck or boost or flyback or any other DC-DC converters, with a fixed output/input ratio, then how do I do if I want a "DC transformer" which give me output = f(input) = input * 3? Or, I don't know, maybe we will never want a DC transformer like this?

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The search term for what you are describing is "proportional dc-dc converter". For example, you can buy high-voltage DC-DC converters, where you put in 5-12V and get 1000 times the voltage out (5K to 12K volts). But for most DC-DC converter uses, the purpose is to get a fixed output voltage even if the input varies.

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Yes, there are converters that have a fixed ratio of input to output such as 48v down to 12 or 4:1.

They are normally referred to as "unregulated".

One application is in telecom servers where they are used to provide galvanic isolation and convert to a lower voltage for distribution in the rack. A switching regulator close to the point of use would then convert the voltage to one that the load needs - for example 1v for a CPU.

This is an example Synqor unregulated converter

You could change a normal converter design act as a ratio converter by using the input voltage as a reference rather than a zener diode or equivalent.

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Most DC-Dc converters are used as regulators. In places which require a fixed voltage supply irrespective of the changes at the source. So having a transformer like action is not really the agenda. Also, the inductors and the capacitors in the converter are designed for a particular voltage and current ratings. So you will not be able to have a converter work satisfactorily in any type of input condition.

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